Some of them have been made into anime, so some scanlators don't care about the manga anymore. Or the manga has been licensed and it takes a whole lot of time for America to translate it. For an instance if a chapter of a manga comes out in Japan on March 20 2008, America will have it translated on June 16 2009.

anyone hear of punch?
i heard it was dropped, but idk. i still hold hope

and they've been uploading a lot of AAA chapters lately.
(i haven't read most of these posts, so idk if anyone has said this already)

anyway. scanlators, if you actually check out their sites, have a LOT of projects. seriously. so my guess is sometimes they either have to put off a few, or drop them entirely, to help with their workload.
plus trying to live their lives outside of the scanlating/translating manga stuffs

so i don't really blame them if projects just disappear. atleast if you're reading it online.
if you're going to the store for it, and it's gone, then i have no idea.

There's this series, Shoulder a Coffin Kuro. The first two volumes have been released in America, but when I noticed that Vol. 2 was released in April, I became suspicious.
The series hadn't ended or anything, so it was weird that a new volume hadn't come out yet.
At first I thought it was Yen Press's fault (They're the American publisher of the series), but once I did some investigating, I saw that Vol. 3 didn't exist in JAPAN either.
So, I'm still waiting on Vol. 3...